Ah, the intelligence quotient—the number everyone pretends not to care about but secretly does. For years, we've assumed that a person's IQ score—commonly used to assess one's general intellectual capacity and make predictions on everything from educational achievement to employment prospects—remained relatively constant over his or her lifetime.

But now, a study that examined the brains of teenagers over several years has revealed for the first time that IQ scores can change dramatically over the course of adolescence, increasing or falling by as much as 21 points in a manner that mirrors specific structural changes in the developing brain.

Before we go any further, it's important to point out that studies examining IQ scores over time have been conducted in the past [PDF]. What makes this research unique, and particularly compelling, is how the scientists coupled these IQ scores with magnetic resonance imaging (MRI) data that allowed them to examine changes in the physical structure of the subjects' brains. That's something that's never been done before—and what they uncovered was surprising.

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"We found a considerable amount of change in how our subjects performed on the IQ tests in 2008 compared to four years earlier," explains Sue Ramsden, first author on the paper documenting the team's results.

Back in 2004, Ramsden and her colleagues recruited 33 boys and girls between the ages of 12 and 16 to participate in the first half of the study. The test subjects all had their IQs tested and the structures of their brains analyzed. The process was repeated for each of the test subjects in 2008.

"Some subjects performed markedly better but some performed considerably worse. We found a clear correlation between this change in performance and changes in the structure of their brains and so can say with some certainty that these changes in IQ are real."

The structural changes Ramsden is referring to boil down to densities of grey matter in the children's brains; increases in a subject's verbal IQ score were found to correlate with an increased grey-matter density in regions of the brain involved in speech and articulation, while increases in the subjects non-verbal IQ score (largely determined through spatial reasoning problems and visual puzzles) corresponded to increased grey-matter density in regions of the brain related to movement.

The researchers explain their findings in the latest issue of Nature:

Our results emphasize the possibility that an individual's intellectual capacity relative to their peers can decrease or increase in the teenage years. This would be encouraging to those whose intellectual potential may improve, and would be a warning that early achievers may not maintain their potential.

Expanding on the implications of her team's results, researcher Cathy Price—who led the study—had this to say:

We have a tendency to assess children and determine their course of education relatively early in life, but here we have shown that their intelligence is likely to be still developing. We have to be careful not to write off poorer performers at an early stage when in fact their IQ may improve significantly given a few more years.

It's analogous to fitness. A teenager who is athletically fit at 14 could be less fit at 18 if they stopped exercising. Conversely, an unfit teenager can become much fitter with exercise.